Semiconductor devices, including, but not limited to, metal-oxide-semiconductor field-effect transistors (MOSFETs), insulated gate bipolar transistors (IGBTs), and diodes, are well known in the art, as are the various applications in which such devices can be employed. Exemplary applications in which semiconductor devices are used include communications systems (e.g., radio frequency (RF) and microwave), automotive electronics, power supplies, high-voltage motors, etc. As a simplistic view, semiconductor devices include a body region, typically formed of a single conductivity type, and means for forming a depletion region (also referred to as a depletion layer) throughout at least a portion of the body. The depletion layer may be formed by applying a positive voltage, VGB, between a gate, or other control terminal, and the body of the device. This gate-body voltage forces positively charged holes away from an interface between the semiconductor and a gate-insulator, thus leaving exposed a carrier-free region of immobile, negatively charged acceptor ions. If the applied gate-body voltage is high enough, a high concentration of negative charge carriers forms in an inversion layer located in a shallow layer proximate the gate-insulator/semiconductor interface for conducting a current through the device.
At least a portion of the body region (e.g., a drain region) may be operated as a drift region for transferring charge carriers due, at least in part, to the effect of an electric field in the semiconductor device when the device is operated in the ON mode. When the device is in the OFF mode, on the other hand, this drift region effectively becomes a depletion region to reduce an electric field strength applied thereon, resulting in an increase in breakdown voltage in the device. The drift region is designed to support a high blocking voltage.
Two important electrical parameters which are often used to characterize the performance of a semiconductor device, particularly power semiconductor devices, are breakdown voltage and on-state resistance, also referred to as on-resistance. Breakdown voltage, VBD, is a parameter of a P-N junction (e.g., in a diode, transistor, etc.) that often defines the largest reverse voltage that can be applied without causing an exponential increase in current flowing through the junction, ultimately damaging the device. On-state resistance, RDSon, of a field-effect transistor (FET) device generally refers to the internal resistance of the device when the device is in its fully conducting (i.e., “on”) state.
For certain applications, such as, but not limited to, power applications, it is generally desirable for a transistor device to have as high a breakdown voltage and as low an on-state resistance as possible. However, breakdown voltage and on-state resistance are mutually exclusive properties of a conventional semiconductor device, since increasing the breakdown voltage rating, for example by incorporating a thicker and lower doped drift region, undesirably leads to higher on-state resistance. Conversely, increasing the doping density in the drift region to thereby reduce the on-state resistance undesirably leads to lower breakdown voltage in the device.
A common method that is well documented in the literature for increasing breakdown voltage in a device without significantly increasing on-resistance involves designing the drift region of the semiconductor device to include a charge balance region, also commonly referred to as a super junction structure or a charge balanced structure. The drift region in a charge balanced semiconductor device is enhanced by extending the depletion region into two dimensions. Conventional methods for fabricating a charge balance structure, however, have substantial disadvantages associated therewith.